The River Porthmos (occasionally called the Porthmos River) is known in Taldor as the "Mighty Porthmos" and is the lifeblood of the nation. Almost the entire length of the river is crowded with merchant boats, trading town and canal entrances. The mouth of the Porthmos is nearly twenty miles across and one of the most heavily protected sections of Taldor's coast as thirty miles upriver lie the Black Cliffs and Oppara. This stretch of river is heavily policed and the river is so busy near Eastport and Westport district and beneath the Grand Bridge that it can be difficult to spot a patch of water between the ships. Those ships not flying the Crown and Lion of Taldor are likely to be detained and taxed by the authorities. From here, the Porthmos heads up and into the Tandak Plains winding its way across the vast grasslands towards its headwaters in the World's Edge Mountains. About sixty miles before the dual headwaters of the river, the Porthmos splits into the North Fork and the South Fork.
While the section of River Porthmos between Oppara and the Inner Sea is well protected, the rest of the river is less so, with some sections being downright dangerous. These dangers include river pirates, a huge nest of crocodiles that dwell where the river splits, and an alarmingly large amount of the usually solitary shambling mounds which dwell along the length of the North Fork.